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Summary: Nested Classes | Constants | Methods

# IBinder

`public interface IBinder`
``

android.os.IBinder

Known indirect subclasses Binder
Binder | Base class for a remotable object, the core part of a lightweight remote procedure call mechanism defined by `IBinder`.

Base interface for a remotable object, the core part of a lightweight remote procedure call mechanism designed for high performance when performing in-process and cross-process calls. This interface describes the abstract protocol for interacting with a remotable object. Do not implement this interface directly, instead extend from `Binder`.

The key IBinder API is `transact()` matched by `Binder#onTransact`. These methods allow you to send a call to an IBinder object and receive a call coming in to a Binder object, respectively. This transaction API is synchronous, such that a call to `transact()` does not return until the target has returned from `Binder#onTransact`; this is the expected behavior when calling an object that exists in the local process, and the underlying inter-process communication (IPC) mechanism ensures that these same semantics apply when going across processes.

The data sent through transact() is a `Parcel`, a generic buffer of data that also maintains some meta-data about its contents. The meta data is used to manage IBinder object references in the buffer, so that those references can be maintained as the buffer moves across processes. This mechanism ensures that when an IBinder is written into a Parcel and sent to another process, if that other process sends a reference to that same IBinder back to the original process, then the original process will receive the same IBinder object back. These semantics allow IBinder/Binder objects to be used as a unique identity (to serve as a token or for other purposes) that can be managed across processes.

The system maintains a pool of transaction threads in each process that it runs in. These threads are used to dispatch all IPCs coming in from other processes. For example, when an IPC is made from process A to process B, the calling thread in A blocks in transact() as it sends the transaction to process B. The next available pool thread in B receives the incoming transaction, calls Binder.onTransact() on the target object, and replies with the result Parcel. Upon receiving its result, the thread in process A returns to allow its execution to continue. In effect, other processes appear to use as additional threads that you did not create executing in your own process.

The Binder system also supports recursion across processes. For example if process A performs a transaction to process B, and process B while handling that transaction calls transact() on an IBinder that is implemented in A, then the thread in A that is currently waiting for the original transaction to finish will take care of calling Binder.onTransact() on the object being called by B. This ensures that the recursion semantics when calling remote binder object are the same as when calling local objects.

When working with remote objects, you often want to find out when they are no longer valid. There are three ways this can be determined:

- The `transact()` method will throw a `RemoteException` exception if you try to call it on an IBinder whose process no longer exists.
- The `pingBinder()` method can be called, and will return false if the remote process no longer exists.
- The `linkToDeath()` method can be used to register a `DeathRecipient` with the IBinder, which will be called when its containing process goes away.

See also:

- `Binder`

## Summary

### Nested classes
`interface` | `IBinder.DeathRecipient` Interface for receiving a callback when the process hosting an IBinder has gone away.

### Constants
`int` | `DUMP_TRANSACTION` IBinder protocol transaction code: dump internal state.
`int` | `FIRST_CALL_TRANSACTION` The first transaction code available for user commands.
`int` | `FLAG_ONEWAY` Flag to `transact(int, Parcel, Parcel, int)`: this is a one-way call, meaning that the caller returns immediately, without waiting for a result from the callee.
`int` | `INTERFACE_TRANSACTION` IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor.
`int` | `LAST_CALL_TRANSACTION` The last transaction code available for user commands.
`int` | `LIKE_TRANSACTION` IBinder protocol transaction code: tell an app asynchronously that the caller likes it.
`int` | `PING_TRANSACTION` IBinder protocol transaction code: pingBinder().
`int` | `TWEET_TRANSACTION` IBinder protocol transaction code: send a tweet to the target object.

### Public methods
`abstract void` | `dump(FileDescriptor fd, String[] args)` Print the object's state into the given stream.
`abstract void` | `dumpAsync(FileDescriptor fd, String[] args)` Like `dump(java.io.FileDescriptor, java.lang.String[])` but always executes asynchronously.
`abstract String` | `getInterfaceDescriptor()` Get the canonical name of the interface supported by this binder.
`static int` | `getSuggestedMaxIpcSizeBytes()` Limit that should be placed on IPC sizes, in bytes, to keep them safely under the transaction buffer limit.
`abstract boolean` | `isBinderAlive()` Check to see if the process that the binder is in is still alive.
`abstract void` | `linkToDeath(IBinder.DeathRecipient recipient, int flags)` Register the recipient for a notification if this binder goes away.
`abstract boolean` | `pingBinder()` Check to see if the object still exists.
`abstract IInterface` | `queryLocalInterface(String descriptor)` Attempt to retrieve a local implementation of an interface for this Binder object.
`abstract boolean` | `transact(int code, Parcel data, Parcel reply, int flags)` Perform a generic operation with the object.
`abstract boolean` | `unlinkToDeath(IBinder.DeathRecipient recipient, int flags)` Remove a previously registered death notification.

## Constants

### DUMP_TRANSACTION

    public static final int DUMP_TRANSACTION

IBinder protocol transaction code: dump internal state.

Constant Value: 1598311760 (0x5f444d50)

### FIRST_CALL_TRANSACTION

    public static final int FIRST_CALL_TRANSACTION

The first transaction code available for user commands.

Constant Value: 1 (0x00000001)

### FLAG_ONEWAY

    public static final int FLAG_ONEWAY

Flag to `transact(int, Parcel, Parcel, int)`: this is a one-way call, meaning that the caller returns immediately, without waiting for a result from the callee. Applies only if the caller and callee are in different processes.

The system provides special ordering semantics for multiple oneway calls being made to the same IBinder object: these calls will be dispatched in the other process one at a time, with the same order as the original calls. These are still dispatched by the IPC thread pool, so may execute on different threads, but the next one will not be dispatched until the previous one completes. This ordering is not guaranteed for calls on different IBinder objects or when mixing oneway and non-oneway calls on the same IBinder object.

Constant Value: 1 (0x00000001)

### INTERFACE_TRANSACTION

    public static final int INTERFACE_TRANSACTION

IBinder protocol transaction code: interrogate the recipient side of the transaction for its canonical interface descriptor.

Constant Value: 1598968902 (0x5f4e5446)

### LAST_CALL_TRANSACTION

    public static final int LAST_CALL_TRANSACTION

The last transaction code available for user commands.

Constant Value: 16777215 (0x00ffffff)

### LIKE_TRANSACTION

    public static final int LIKE_TRANSACTION

IBinder protocol transaction code: tell an app asynchronously that the caller likes it. The app is responsible for incrementing and maintaining its own like counter, and may display this value to the user to indicate the quality of the app. This is an optional command that applications do not need to handle, so the default implementation is to do nothing.

There is no response returned and nothing about the system will be functionally affected by it, but it will improve the app's self-esteem.

Constant Value: 1598835019 (0x5f4c494b)

### PING_TRANSACTION

    public static final int PING_TRANSACTION

IBinder protocol transaction code: pingBinder().

Constant Value: 1599098439 (0x5f504e47)

### TWEET_TRANSACTION

    public static final int TWEET_TRANSACTION

IBinder protocol transaction code: send a tweet to the target object. The data in the parcel is intended to be delivered to a shared messaging service associated with the object; it can be anything, as long as it is not more than 130 UTF-8 characters to conservatively fit within common messaging services. As part of `Build.VERSION_CODES#HONEYCOMB_MR2`, all Binder objects are expected to support this protocol for fully integrated tweeting across the platform. To support older code, the default implementation logs the tweet to the main log as a simple emulation of broadcasting it publicly over the Internet.

Also, upon completing the dispatch, the object must make a cup of tea, return it to the caller, and exclaim "jolly good message old boy!".

Constant Value: 1599362900 (0x5f545754)

## Public methods

### dump

    public abstract void dump (FileDescriptor fd,
                    String[] args)

Print the object's state into the given stream.

Parameters
`fd` | `FileDescriptor`: The raw file descriptor that the dump is being sent to. This value cannot be `null`.
`args` | `String`: additional arguments to the dump request. This value may be `null`.

Throws
`RemoteException` |

### dumpAsync

    public abstract void dumpAsync (FileDescriptor fd,
                    String[] args)

Like `dump(java.io.FileDescriptor, java.lang.String[])` but always executes asynchronously. If the object is local, a new thread is created to perform the dump.

Parameters
`fd` | `FileDescriptor`: The raw file descriptor that the dump is being sent to. This value cannot be `null`.
`args` | `String`: additional arguments to the dump request. This value may be `null`.

Throws
`RemoteException` |

### getInterfaceDescriptor

    public abstract String getInterfaceDescriptor ()

Get the canonical name of the interface supported by this binder.

Returns
`String` | This value may be `null`.

Throws
`RemoteException` |

### getSuggestedMaxIpcSizeBytes

    public static int getSuggestedMaxIpcSizeBytes ()

Limit that should be placed on IPC sizes, in bytes, to keep them safely under the transaction buffer limit.

Returns
`int` |

### isBinderAlive

    public abstract boolean isBinderAlive ()

Check to see if the process that the binder is in is still alive.

Returns
`boolean` | false if the process is not alive. Note that if it returns true, the process may have died while the call is returning.

### linkToDeath

    public abstract void linkToDeath (IBinder.DeathRecipient recipient,
                    int flags)

Register the recipient for a notification if this binder goes away. If this binder object unexpectedly goes away (typically because its hosting process has been killed), then the given `DeathRecipient`'s `DeathRecipient#binderDied` method will be called.

You will only receive death notifications for remote binders, as local binders by definition can't die without you dying as well.

Parameters
`recipient` | `IBinder.DeathRecipient`: This value cannot be `null`.
`flags` | `int`

Throws
`RemoteException` | if the target IBinder's process has already died.

See also:

- `unlinkToDeath(IBinder.DeathRecipient, int)`

### pingBinder

    public abstract boolean pingBinder ()

Check to see if the object still exists.

Returns
`boolean` | Returns false if the hosting process is gone, otherwise the result (always by default true) returned by the pingBinder() implementation on the other side.

### queryLocalInterface

    public abstract IInterface queryLocalInterface (String descriptor)

Attempt to retrieve a local implementation of an interface for this Binder object. If null is returned, you will need to instantiate a proxy class to marshall calls through the transact() method.

Parameters
`descriptor` | `String`: This value cannot be `null`.

Returns
`IInterface` |

### transact

    public abstract boolean transact (int code,
                    Parcel data,
                    Parcel reply,
                    int flags)

Perform a generic operation with the object.

Parameters
`code` | `int`: The action to perform. This should be a number between `FIRST_CALL_TRANSACTION` and `LAST_CALL_TRANSACTION`.
`data` | `Parcel`: Marshalled data to send to the target. Must not be null. If you are not sending any data, you must create an empty Parcel that is given here. This value cannot be `null`.
`reply` | `Parcel`: Marshalled data to be received from the target. May be null if you are not interested in the return value. This value may be `null`.
`flags` | `int`: Additional operation flags. Either 0 for a normal RPC, or `FLAG_ONEWAY` for a one-way RPC.

Returns
`boolean` | Returns the result from `Binder#onTransact`. A successful call generally returns true; false generally means the transaction code was not understood.

Throws
`RemoteException` |

### unlinkToDeath

    public abstract boolean unlinkToDeath (IBinder.DeathRecipient recipient,
                    int flags)

Remove a previously registered death notification. The recipient will no longer be called if this object dies.

Parameters
`recipient` | `IBinder.DeathRecipient`: This value cannot be `null`.
`flags` | `int`

Returns
`boolean` | `true` if the recipient is successfully unlinked, assuring you that its `DeathRecipient#binderDied` method will not be called; `false` if the target IBinder has already died, meaning the method has been (or soon will be) called.

Throws
`NoSuchElementException` | if the given recipient has not been registered with the IBinder, and the IBinder is still alive. Note that if the recipient was never registered, but the IBinder has already died, then this exception will not be thrown, and you will receive a false return value instead.

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